Mach Zehnder interferometers (MZIs) often are used to modulate the amplitude or intensity of an optical signal. MZIs are most often deployed as modulators in digital links. However, they also have application in analog links, such as in multi- or sub-octave remote antenna links.
Generally, deployed MZI modulators are monolithic devices formed from an electro-optically active material such as lithium niobate. The MZIs have two couplers and two arms. Electrodes are provided for controlling the electric field in the region of the two arms. An input signal is divided between the two arms by a first coupler, with the divided signal passing through the two arms, and then recombined by the second coupler. Often, the arms have equal length and the couplers are 50/50 couplers.
During operation, the relative phase of the divided input signals is controlled by some type of modulation in one or both arms. The intensity of the output signal thus depends on the interference of the divided input signals after recombination. In this type of device a phase difference of π, between the output signals of the arms of the interferometer, is generally required to change the output intensity from an “on” state to an “off” state, and the associated voltage required for such a change is referred to as Vπ.
FIG. 1 shows a variant of the classical MZI design, which combines features of a MZI and a ring resonator. This device and its behavior are described in U.S. Patent Application Publication No. 2001/0004411 A1, filed on Jun. 21, 2001, and in an article by Yariv, A., Universal Relations for Coupling of Optical Power Between Microresonators and Dielectric Waveguides, Electronics Lett. Vol. 36, No. 4, p. 321, (2000).
In this resonant modulator device, a balanced MZI was interposed in the path of a waveguide ring resonator 124, using a waveguide cross 130 and 3 dB directional couplers 126, 128 into and from the MZI. The differential phase shift Δφ distributed between arms 120 and 122 is +Δφ/2 and −Δφ/2, respectively. In a “balanced” (also referred to as “symmetrical” or “push-pull”) arrangement, a voltage of +V/2 is applied to the electrode associated with one arm and a voltage of −V/2 is applied to the electrode associated with the other arm. It has been shown that, for this configuration, a very small (compared to π) change in the interferometer phase, Δφ, can switch the output state from on to off. Likewise the voltage required for on-off operation is very small compared to the traditional Vπ required for a conventional MZI. The conditions that achieve such operation are referred to as “critical coupling”.